VSC-converter

ABSTRACT

In a VSC-converter for converting direct voltage to alternating voltage and conversely at least one phase leg ( 2 ) is arranged according to a NPC-connection. At least two of the four current valves ( 3-6 ) and the clamping rectifying members ( 10, 11 ) thereof are arranged close to each other and so that commutation currents generated on commutation of the converter therein will flow in substantially opposite directions.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a VSC-converter for converting directvoltage into alternating voltage and conversely and which comprises atleast one phase leg with an NPC-connection, i.e. four current valvesconnected in series, which consist of at least one semiconductor deviceof turn-off type and a first rectifying member connected inanti-parallel therewith, in which a point on the phase leg between twoinner valves of the series connection is intended to be connected to aphase of an alternating voltage network and the opposite ends of thephase leg are intended to be connected to a pole conductor each of adirect voltage network or a direct voltage intermediate link, and inwhich a series connection of two second so called clamping rectifyingmembers directed in the same direction with respect to said seriesconnection as the first rectifying members are connected between on onehand a point between one of the outer valve of the series connection andthe adjacent inner valve and on the other a point between the other ofthe outer valves in the series connection and the adjacent inner valve,and a midpoint between the two clamping rectifying members is connectedto a zero potential defined by capacitors connected in series betweensaid pole conductors.

Such a VSC-converter for connection between a direct voltage network andan alternating voltage network has recently become known through thethesis “PWM and control of two and three level High Power Voltage SourceConverters” by Anders Lindberg, Kungliga Tekniska Högskolan, Stockholm,1995, in which publication a plant for transmitting electric powerthrough a direct voltage network for high voltage direct current (HVDC)while utilizing such converters is described. Before the issuance ofsaid thesis plants for transmitting electric power between a directvoltage network and an alternating voltage network have been based uponthe use of line-commutated CSC(Current Source Converters)-converters instations for power transmission. However, in this thesis a totally newconcept is described, which is based on the use of VSC(Voltage SourceConverter)converters for forced commutation instead for transmittingelectric power between a direct voltage network being voltage-stifftherethrough, in the present case for high voltage direct current, andalternating voltage networks connected thereto, which offers severalimportant advantages with respect to the use of line-commutatedCSC-converters in HVDC, of which it may be mentioned that theconsumption of active and reactive power may be controlled in dependencyof each other and there is no risk of commutation failures in theconverter and thereby no risk of commutation failures between differentHVDC links, which may take place in line-commutated CSC's. Furthermore,there is a possibility to feed a weak alternating voltage network or anetwork without a generation of its own (a dead alternating voltagenetwork). There are also further advantages.

The invention is not restricted to this application, but the convertermay just as well be intended for conversion in a SVC, in which case thedirect voltage network is replaced by a DC-intermediate link.“Network”is also to be given a very broad sense, and it does not have tobe a question about any such networks in the real meaning of that word.However, the problems of the invention will now be illuminated forexactly this application, although the problem is common to allconceivable applications within the scope of the invention. When using aconverter of the type mentioned in the introduction instead of acurrent-stiff CSC-converter a new problem not present in the latter iscreated, which will now be explained by reference to FIGS. 1, 2 and 3.Firstly, it may be mentioned that the advantage of using a so calledNPC(Neutral Point Clamped)-connection, which accordingly means that thephase terminal may be provided with positive and negative voltage fromthe respective pole conductor as well as zero voltage, with respect to aso called two-pulse bridge, or in the case of three phases a so calledsix-pulse bridge, is that a lower switching frequency may used, whichresults in lower losses and a higher efficiency. However, this stillsuffers from said problems not yet described. A VSC-converter 1 of thetype defined in the introduction is shown in FIG. 1 and it has a phaseleg 2 with four current valves 3-6 connected in series, which eachconsists of at least one semiconductor device of turn-off type, such asan IGBT 7, and a first rectifying member in the form of a diode 8connected in anti-parallel therewith. A point on the phase leg betweenthe two inner valves 4, 5 in the series connection is intended to beconnected to a phase 9 of an alternating voltage network. A seriesconnection of two second so called clamping rectifying members 10, 11 inthe form of diodes directed in the same direction with respect to saidseries connection as the first rectifying members are connected betweenon one hand a point 12 between one outer valve 3 in the seriesconnection and the inner valve 4 adjacent thereto and on the other apoint 13 between the outer valve 6 of the series connection and theinner valve 5 adjacent thereto with a midpoint 14 between the twoclamping rectifying members connected to a zero potential 15 defined bycapacitors 19, 20 connected in series between a positive pole conductor16 and a negative pole conductor 17 of the direct voltage network 18. Itis illustrated in FIGS. 2 and 3 what is happening upon commutation, i.e.when the voltage output on the phase terminal 9 is changed. It isimagined that in the case shown in FIG. 2 current flows from thenegative pole conductor 17 of the direct voltage network through thediodes of valves 5 and 6 to the phase terminal 9, which then gets avoltage −U_(d) of the pole conductor 17. The semiconductor devices 7 ofthe current valves 3 and 4 are turned off. If a commutation now takesplace, so that the semiconductor device in the current valve 4 is turnedon, the current will then be commutated to go through the clamping diode10 and the current valve 4 to the phase terminal 9, which then receiveszero potential. This will result in a commutation current according tothe loop 21, which is shown in FIG. 2. It is illustrated in FIG. 3 whatis happening if the first semiconductor devices of the current valves 5and 6 are turned on and thereby the current flows from the phaseterminal 9 to the pole conductor 17 and thereby the phase terminal hasthe potential −U_(d) and then a commutation takes place, so that thesemiconductor device in the current valve 6 is turned off and thecurrent is instead lead through the current valve 5 and the clampingdiode 11 to the point 14 and through the capacitor 20 to the poleconductor 17, so that the phase output 9 receives zero potential. Acommutation current according to a smaller commutation loop 22 is thenresulting. The commutation current will flow in the respectivecommutation loop in the opposite direction should the commutation takeplace in the opposite order to the one described. Similar commutationloops, i.e. a large and a small loop also exist upon commutation throughthe upper capacitor 19. The commutation times of such a VSC-converterare short and comparatively large time differential coefficients of thecurrent are created, which makes it desired to lower the inductance inthe commutation circuit, i.e. in the very phase leg and the lineconnected between the poles of the direct voltage network withcapacitors for defining the direct voltage, to a level being as low aspossible so as to avoid unnecessarily high over-voltages and therebylosses upon said commutation.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a VSC-converter of thetype defined in the introduction, in which the problem mentioned abovehas been solved in a satisfying way.

This object is according to the invention obtained by the fact that atleast two of the current valves and the clamping rectifying members arearranged close to each other and so that commutation currents generatedupon commutation of the converter therein will flow in substantiallyopposite directions.

By ensuring that such commutation currents flow close to each other andin opposite directions in this way the inductance of the commutationcircuit will be reduced considerably, since the currents passing eachother will generate magnetic fields rotating in the opposite directionand partially cancelling each other out. The commutation losses maythereby be reduced to a low, acceptable level.

According to a preferred embodiment of the invention the current valvesand the clamping rectifying members are arranged in stacks arranged inpairs and so that commutation currents generated upon commutation of theconverter will flow in substantially opposite directions in stacksbelonging to the same pair. Such an arrangement of stacks in pairs leadsto a low inductance value of the commutation circuit and thereby lowcommutation losses.

According to another preferred embodiment of the invention each currentvalve and clamping rectifying member is formed by one or several stacksof substantially identical units connected in series, and the stacks aredesigned with a large width and adjacent stacks are arranged at shortmutual distances. By utilizing the so-called ribbon cable technique inthis way a low inductance of the commutation circuit with the advantagesmentioned above as a consequence thereof is obtained.

According to another preferred embodiment of the invention for at leastone of the following three pairs the two parts included therein arearranged close to each other and so that commutation currents generatedupon commutation of the converter in those belonging to the same pairwill flow in substantially opposite directions: a) a first outer currentvalve and the clamping rectifying member connected between the secondouter current valve and the inner current valve adjacent thereto, b) thesecond outer current valve and the clamping rectifying member connectedbetween the first outer current valve and the inner current valveadjacent thereto and c) the two inner current valves. These differentpairs will belong to the larger commutation circuit 21 shown in FIG. 2or the opposite large commutation circuit going through the currentvalves 3, 4, 5 and the clamping rectifying member 11. It is ofparticular importance that the inductance of the large commutationcircuit is reduced in this way, since it is this inductance that is thehighest of the two commutation circuits as a consequence of the longerway to flow for the commutation current in this circuit and the largerarea enclosed by such a circuit.

According to another preferred embodiment of the invention, whichconstitutes an advantageous further development of the embodiment lastmentioned, all three pairs a), b) and c) have their pair parts arrangedclose to each other and with said commutation current in each pairoppositely directed when they flow in a large commutation loop throughan outer current valve, the inner current valve adjacent thereto, thesecond inner current valve and the clamping rectifying member connectedto the latter. This results in a particularly low inductance of the twolarge commutation circuits and commutation losses at a low acceptablelevel.

According to another preferred embodiment of the invention the fourcurrent valves and the clamping rectifying members are arrangedsubstantially in a row one after the other with a clamping rectifyingmember arranged between each outer valve of the series connection andthe inner valve connected thereto. By arranging the four current valvesand the clamping rectifying members in this way in a row the commutationcircuits 21, 22 mentioned above may be made small, i.e. the areaenclosed by the respective commutation circuit gets very small, and theinductance of the commutation circuit is reduced with reducing areaenclosed by the circuit. The commutation circuits will also by such anarrangement in a row be very narrow and the inductance thereof willthereby be reduced and the currents passing each other will generatemagnetic fields rotating in opposite directions and partially cancellingeach other out. The commutation losses will thereby be brought down to alow acceptable level.

According to another preferred embodiment of the invention the currentvalves and the clamping rectifying members are adapted to extendsubstantially perpendicularly to the extension direction of said row.The row may hereby be made comparatively short and the length of thecommutation circuit will thereby be reduced, i.e. the conductingdistance of the current will be shorter, which reduces the inductance,since this is proportional to this length.

According to another preferred embodiment of the invention one outercurrent valve in said series connection and the inner current valvelocated most remotely with respect thereto are arranged substantially inparallel with each other, and the two other current valves are arrangedsubstantially in parallel with each other and making an angle ofsubstantially 180° with respect to the two valves first mentioned. It ishereby obtained that the commutation circuits get as narrow as possible,i.e. the area enclosed thereby gets as small as possible and thereby theinductance low.

According to another preferred embodiment of the invention the twoclamping rectifying members are arranged with the conducting directionsthereof making an angle of substantially 180° with each other. Thismeans that the two commutation circuits of the respective half of aNPC-connection may be closed through the respective clamping rectifyingmember in such a way that the circuit will be as short as possible andencloses a very small area.

According to another preferred embodiment of the invention, whichconstitutes a further development of the embodiment last described, therespective clamping rectifying member is arranged with the conductingdirection thereof making an angle of substantially 180° with respect tothe conducting direction of the first rectifying member in the outercurrent valve adjacent thereto. Lengths and areas enclosed of and by thecommutation circuits with minimal dimensions are obtained thereby,especially the smaller commutation circuit with the advantages appearingabove as a consequence.

According to another preferred embodiment of the invention said row isfolded by substantially 180° in the region of said phase terminal, sothat the outer current valves come close to each other and the phase leggets compact. The VSC-converter may in this way be placed in a veryvolume-saving way, and in the case of an arrangement of said row invertical direction the height thereof may in this way be reduced.

According to another preferred embodiment of the invention saidcapacitors are two to the number and one of them is connected to eachend of said row with one plate to the outer current valve located thereand the other plate to a clamping rectifying member adjacent thereto.This embodiment in combination with the preceding one means that in thecase of a standing row the two capacitors will be placed adjacent toeach other in the ground plane. This location of the capacitors makesthe commutation circuits short, in particular the smaller commutationcircuits.

According to another preferred embodiment of the invention each currentvalve consists of a plurality of semiconductor devices of turn-off typeconnected in series and rectifying members connected in anti-paralleltherewith arranged so that they form a substantially U-shape. Theinductance in the respective current valve is in this way reduced thanksto the currents passing each other.

According to another preferred embodiment of the invention, whichconstitutes a preferred further development of the previous embodiment,the legs of said U are thick and the distance therebetween comparativelysmall for reducing the inductance of the respective current valve. Byconstructing the respective current valve in this way according to theribbon cable principle the inductance may be reduced to low levels.

According to another preferred embodiment of the invention the differentconnections between the current valves and the clamping rectifyingmembers and capacitors of the converter are achieved by low inductancerails, i.e. thin rails having a comparatively large width transverselyto the longitudinal extension thereof. The inductance of the commutationcircuits is hereby reduced further.

According to another preferred embodiment of the invention the converteris intended to be connected to a direct voltage network for high voltagedirect current (HVDC). A series connection of a comparatively highnumber, well about thirty, of semiconductor devices and thereby ofrectifying members connected in anti-parallel therewith is required forforming a current valve in this application, so that the invention isespecially well suited for exactly this application.

Further advantages as well as advantageous features of the inventionwill appear from the following description and the other dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description ofpreferred embodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a schematic circuit diagram illustrating the construction of aVSC-converter of the type according to the invention, but this has onlybeen shown for one phase,

FIGS. 2 and 3 are views corresponding to FIG. 1 illustrating how acommutation circuit for a commutation current may be formed for twodifferent possible commutations,

FIG. 4 illustrates schematically the construction of a VSC-converteraccording to a first preferred embodiment of the invention,

FIG. 5 illustrates a VSC-converter of the type illustrated in FIG. 4,but all three phase legs are shown here,

FIGS. 6 and 7 are views illustrating a VSC-converter according to saidpreferred embodiment of the invention, in which the figures show it fromdifferent directions,

FIG. 8 illustrates schematically the construction of a VSC-converteraccording to a third preferred embodiment of the invention,

FIG. 9 illustrates schematically a VSC-converter according to a fourthpreferred embodiment of the invention, and

FIG. 10 illustrates an advantageous embodiment and arrangement ofconducting rails of the VSC-converter according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 has already been discussed, and it may here be noted that itschematically illustrates the construction of a VSC-converter ofNPC-type connected between a direct voltage network for high voltagedirect current (HVDC=High Voltage Direct Current), or only a DCintermediate link (SVC operation), and a three-phase alternating voltagenetwork, although only one phase 9 and a phase leg 2 are shown in FIG.1, but two additional such phase legs are in practice connected inparallel with the phase leg 2 between the two poles 16 and 17 of thedirect voltage network. Each phase leg of the converter has four socalled current valves, which consist of units of breakers 7 of turn-onand turn-off type connected in series, preferably in the form of IGBT's,and rectifying members connected in anti-parallel therewith, i.e.members being conducing in one direction and blocking in the oppositedirection, in the form of diodes 8. A large number of such units andthereby IGBT's may be connected in series in one single valve so as tobe turned on and turned off simultaneously so as to act as one singlebreaker or switch, where the voltage over the valve is distributed onthe different breakers connected in series. The control of the breakerstakes place through pulse width modulation (PWM). Furthermore, theclamping diodes 10 and 11 already described are used for assisting inclamping or fixing the voltage over valves not conducting, so that whenfor example the valves 5 and 6 are turned on and the voltage -U_(d) isapplied on the phase terminal 9 the diode of the current valve 3 and thediode 10 “clamp” the point 12 to a potential between 0 and +U_(d) in away known.

The inductance problem present in a converter of this type havethoroughly been discussed in the introductory portion of the descriptionand are not to be repeated here, but it may be noted that as aconsequence of the high switching frequencies used in this form ofconverters, one or a few kHz, opposite to the switching frequencies inthe order of 50 Hz of CSC-converters, the time differential coefficientsof the commutation current will be very high, and it is thereforeessential to try to reduce the inductance of the commutation circuit soas to reduce the over-voltages and the losses resulting after all. Someembodiments of converters according to the invention falling within thescope of the invention will now be described with reference to FIGS.4-9.

It is shown in FIG. 4 how the different current valves 3-6 and clampingdiodes 10, 11 are arranged substantially in a row one after the other,in which a clamping diode is placed between the respective outer valvein the series connection and the inner valve connected thereto, such asthe diode 11 between the current valves 5 and 6. The current valves andthe clamping diodes are arranged to extend substantially perpendicularlyto the direction of extension of the row. As appears, the current valves3 and 5 are directed substantially in parallel with each other, whilethe current valves 4 and 6 are directed substantially in parallel witheach other, but making an angle of substantially 180° with the twocurrent valves first mentioned. Furthermore, the two clamping diodesdirected with their conducting directions in opposite directions andwith the conducting direction in the opposite direction to theconducting direction of the diode 8 in the outer current valves 3 and 6,respectively, adjacent thereto. The different connections between thecurrent valves and the clamping diodes as well as the capacitors 19, 20are achieved by low inductance rails 23, i.e. thin rails having acomparatively large width transversely to the longitudinal extensionthereof. This shape of these rails appears better from FIGS. 5-7. In theembodiment according to FIG. 4 the connections 23 between the currentvalves and the clamping diodes for at least the main part of the totallength thereof are substantially in parallel with the longitudinaldirection of said row.

The commutation circuits get through this construction of the currentvalve a comparatively short length and will enclose a small area, sothat the inductance thereof will be low. This is understood by lookingon FIG. 4, since the corresponding commutation circuits in FIGS. 2 and 3will run according to the following: If we consider the largecommutation circuit to start on the top, it runs there, as seen in FIG.4, to the left through the clamping diode 10, then downwardly to thecurrent valve 4, through the semiconductor device thereof and thendownwardly to the phase terminal 9, further to the current valve 5 andthrough the semiconductor device thereof to the left and then straightdownwardly to the current valve 6, through the semiconductor devicethereof to the right, then to the capacitor 20 and then to the left andafter that straight upwardly through the neutral rail 23 to the clampingdiode 10. Thus, this circuit gets very narrow and encloses a small area.The short commutation circuit will run from the left to the rightthrough the clamping diode 11, then to the capacitor 20, and after thatto the left through the diode of the current valve 6 for runningupwardly to the diode 11 again. There are also corresponding loopsthrough the upper capacitor 19.

It is illustrated in FIG. 5 what a converter connected to a three-phasealternating voltage network with the construction according to FIG. 4for each phase may look like. Thus, three phase terminals 9, 9′, 9″, onefor each phase leg 2, 2′, 2″, exist here. On the top to the left in FIG.5 the construction of a current valve or clamping diode is illustrated,and it appears that these consist of a number of units connected inseries, which are arranged so that they form substantially a U-shape.The legs 24 of the U are thick and the distance therebetween iscomparatively short for reducing the inductance of the respectivecurrent valve or clamping diode. Accordingly, “clamping diode” or“clamping rectifying member” are also to be interpreted as it may beformed by an amount of such diodes or members connected in series.

In the embodiment shown in FIG. 5 the phase leg is stretched anddirected substantially vertically, but it is also possible to fold therow, preferably at the phase terminal, and preferably by an angle ofsubstantially 180°, so that the phase leg with the capacitors gets theappearance appearing from FIGS. 6 and 7, in which the second parts areshown in these two Figures, but from different directions. Hereby theconverter becomes very compact, and in the case it is arranged invertical direction the height may be reduced substantially, and the twocapacitors may then be arranged at the ground level.

A very preferred embodiment of the invention is illustrated in FIG. 8,in which the four current valves and clamping rectifying members arearranged in three pairs of stacks arranged close to each other with thedirection of the commutation currents flowing therethrough oppositelydirected for the large commutation loop shown in FIG. 2 and the largecommutation loop running through the upper current valve shown in FIG.2. More exactly, the current valve 3 and the clamping rectifying member11 form a pair, the current valve 6 and the clamping rectifying member10 another pair, while the current valve 5 and the current valve 4 forma third pair. Such an arrangement of the current valves and the clampingrectifying members in stacks arranged in pairs with the direction of thecommutation current mentioned in the large commutation loop results in avery low inductance for this commutation loop and the commutation losseswill thereby be kept at a low level. It is then particularlyadvantageous if these stacks belonging to the same pair are arrangedclose to each other and are given a large width so as to utilize theso-called ribbon cable principle.

A converter according to a fourth preferred embodiment is schematicallyillustrated in FIG. 9. Each current valve and clamping rectifyingmember, respectively, is in this converter formed by two stacks, inwhich these are shown from the top and the direction of the commutationcurrent therethrough is shown by X (into the drawing) and. (out of thedrawing). The commutation currents will in this way flow in oppositedirections for an amount of adjacent stacks and the inductance of thecommutation circuits may thereby be reduced. With respect to the largecommutation loop, it appears that also here 3 and 11 as well as 4 and 5form pairs, while 10 and 6 are not adjacent to each other. Thecommutation currents through the rails 25 and 26 will over large partsof the extension thereof run in opposite directions, so that theinductance also of that part of the commutation circuits will be low.

It is illustrated in FIG. 10 how these rails are preferably designed andarranged, namely so that they are wide and arranged on a small mutualdistance for utilizing the so-called ribbon cable principle.

Converters of the type according to the invention may be used both fortransmitting active and reactive power by generating losses being smallin this context.

The invention is of course not in any way restricted to the preferredembodiments described above, but many possibilities to modificationsthereof will be apparent to a man skilled in the art without departingfrom the basic idea of the invention, such as this appears from theclaims.

For example “substantially perpendicular to” and “making an angle ofsubstantially 180° with respect to” in the claims are to be given abroad sense and means that the current valves and the clampingrectifying members are adapted to extend transversely to thelongitudinal extension of said row and that these are directed in mainlyopposite directions, respectively, and that the angles lyingcomparatively close to these angles are also covered.

What is claimed:
 1. A VSC-converter for converting direct voltage intoalternating voltage and conversely and which comprises at least onephase leg with an NPC-connection, four current valves connected inseries, including at least one semiconductor device of turn-off type anda first rectifying member connected in anti-parallel therewith, in whicha point on the phase leg between two inner valves of the seriesconnection is for connection to a phase of an alternating voltagenetwork and opposite ends of the phase leg are for connection to a poleconductor each of a direct voltage network or a direct voltageintermediate link, and in which a series connection of two secondclamping rectifying members directed in the same direction with respectto said series connection as the first rectifying members are connectedbetween on one hand to a point between one of the outer valves of theseries connection and the adjacent inner valve and on the other hand toa point between the other of the outer valves in the series connectionand the adjacent inner valve, and a midpoint between the two clampingrectifying members is connected to a zero potential defined bycapacitors connected in series between said pole conductors, wherein atleast two of the current valves and the clamping rectifying members arearranged in relatively close proximity to each other and are orientedwith respect to each other so that commutation currents generate uponcommutation of the converter therein will flow in substantially oppositedirections.
 2. The converter according to claim 1, wherein the currentvalves and the clamping directing members are arranged in stacksarranged in pairs and so that commutation currents generated uponcommutation of the converter will flow in substantially oppositedirections in stacks belonging to the same pair.
 3. The converteraccording to claim 1, wherein each current valve and clamping rectifyingmember is formed by one or several stacks of substantially identicalunits connected in series, and said stacks are formed with a large widthand adjacent stacks are arranged at short mutual distances.
 4. Theconverter according to claim 1, comprising rails connecting the currentvalves and the clamping rectifying members to each other, and the railsbeing arranged that commutation currents generated upon commutation ofthe converter flow in substantially opposite directions in adjacent suchrails over at least parts of the extension thereof.
 5. The converteraccording to claim 1, wherein for at least one of the following threepairs, the two parts included therein are arranged close to each otherand so that commutation currents generated upon cummutation of theconverter in those belonging to the same pair flow in substantiallyopposite directions: a) the first outer current valve and the clampingrectifying member connected between the second outer current valve andthe inner current valve adjacent thereto, b) the second outer currentvalve and the clamping rectifying member connected between the firstouter current valve and the inner current valve adjacent thereto and c)the two inner current valves.
 6. The converter according to claim 5,wherein all the pairs a), b) and c) have pair parts arranged close toeach other and with said commutation current in each pair oppositelydirected when they flow in a large commutation loop through an outercurrent valve, the inner current valve adjacent thereto, the other innercurrent valve and the clamping rectifying member connected to thelatter.
 7. The converter according to claim 1, wherein the four currentvalves and the clamping rectifying members are arranged substantially ina row one after the other with a clamping rectifying member arrangedbetween each outer valve of the series connection and the inner valveconnected thereto.
 8. The converter according to claim 7, wherein thecurrent valves and the clamping rectifying members are arranged toextend substantially perpendicularly to the direction of extension ofsaid row.
 9. The converter according to claim 7, wherein one outercurrent valve in said series connection and the inner current valvelocated most remotely with respect thereto are arranged substantially inparallel with each other, and that the two other current valves arearranged substantially in parallel with each other and making an angleof substantially 180° with respect to the two valves first mentioned.10. The converter according to claim 1, wherein the two clampingrectifying members are arranged with their conducting directions makingan angle of substantially 180° with each other.
 11. The converteraccording to claim 10, wherein the respective clamping rectifying memberis arranged with the conducting direction thereof making an angle ofsubstantially 180° with the conducting direction of the first rectifyingmember in the outer current valve adjacent thereto.
 12. The converteraccording to claim 7, wherein said row is folded by substantially 180°in the region of said phase terminal, so that the outer current valvescome close to each other and the phase leg gets compact.
 13. Theconverter according to claim 1, wherein each current valve includes aplurality of semiconductor devices of turn-off type connected in seriesand rectifying members connected in anti-parallel therewith arranged sothat they form a substantially U-shape.
 14. The converter according toclaim 13, wherein the legs of said U are relatively thick and thedistance therebetween comparatively small for reducing the inductance ofthe respective current valve.
 15. The converter according to claim 1,wherein the different connections between the current valves and theclamping rectifying members and capacitors (19, 20) of the converter areformed by relatively low inductance rails, with a comparatively largewidth transversely to the longitudinal extension thereof.
 16. Theconverter according to claim 7, wherein the connections between thecurrent valves and the clamping rectifying members over at least themain part of the total length thereof extend substantially in thelongitudinal direction of said row.
 17. The converter according to claim7, wherein said capacitors are two in number and one of them isconnected to each end of said row with one plate to the outer currentvalve and the other plate to the clamping rectifying member adjacentthereto.
 18. The converter according to claim 17, wherein one of thecapacitors is connected to the opposite end of the current valve and theclamping rectifying member with respect to the second capacitor withrespect to the conducting direction.
 19. The converter according toclaim 1, wherein the clamping rectifying members are diodes.
 20. Theconverter according to claim 1, wherein the first rectifying members arediodes.
 21. The converter according to claim 1, wherein thesemiconductor devices are IGBTs.
 22. The converter according to claim 1,including a plurality of phase legs for connection to a plurality ofphases of an alternating voltage network.
 23. The converter according toclaim 1, for connection to a direct voltage network for high voltagedirect current (HVDC).